The present invention relates to a glenoid component for a shoulder prosthesis and to a complete shoulder prosthesis incorporating such a component.
In the domain of shoulder prostheses, it is known, for example from U.S. Pat. No. 3,978,528, to constitute an inverted prosthesis in which a convex articular surface is secured to the glenoid cavity, while a concave articular surface is secured to the humerus, the cooperation of these surfaces making it possible to re-create a joint of the shoulder. In this type of prosthesis, a base may be used which is fixed on the glenoid cavity and on which is mounted an element forming the convex articulation surface.
The design of inverted prostheses of the type mentioned above responds to the need to restore the indolence and the function of the shoulder joint in the cases of massive destruction of the cover of the elements ensuring rotation. In that case, the raising of the arm is ensured solely by the deltoid muscle. It is therefore important that the lever arms corresponding to the points of application of the forces undergone by the joint be determined with high precision in order to limit as much as possible the efforts that a patient must develop in order to raise an arm fitted with a shoulder prosthesis. In effect, the known prostheses sometimes cause discomfort for the patients.
It is a more particular object of the present invention to overcome these drawbacks by proposing a glenoid component of a shoulder prosthesis which takes maximum advantage of the muscular tension, reducing the efforts necessary for raising the arm.
In this spirit, the invention relates to a glenoid component of a shoulder prosthesis which comprises a base, adapted to be immobilized on the glenoid cavity of a shoulder, and an element forming a convex articulation surface adapted to cooperate with a humeral component of the prosthesis, this element being provided to be mounted on this base. This component is characterized in that the element forming the convex surface is adapted to be mounted on the base in a plurality of positions corresponding respectively to different positions of the convex articulation surface with respect to the glenoid cavity.
Thanks to the invention, the different possible positions of the convex articulation surface with respect to the glenoid cavity may be used for varying, in pre-operatory phase, the position of this surface, which makes it possible to adjust with precision the geometry of the joint thus re-created.
According to advantageous but non-obligatory aspects of the invention, this component incorporates one or more of the following characteristics:
The element forming an articulation surface is constituted by a piece in the form of a portion of sphere forming the articulation surface and by a plate adapted to support this piece and provided with means for positioning and fixation with respect to the base.
Means are provided for indexation of the position of the element forming the convex articulation surface with respect to the base. These indexation means, i.e. for incremental positioning, may comprise an extension projecting from the afore-mentioned element, as well as a plurality of housings provided on the base for the selective reception of this extension. Depending on the housing in which the afore-mentioned extension is received, different positions of the convex articulation surface with respect to the glenoid cavity are obtained. The afore-mentioned housings are advantageously defined by the edges of a notch made in a web of the base intended to be applied against the glenoid cavity. In addition, the indexation means may be provided to comprise orifices for passage of screws kinematically linked to the element forming the convex articulation surface, these orifices being distributed in the vicinity of the afore-mentioned housings in order to receive these screws, as a function of the position of the extension in one of these housings.
When the element forming the extension is bipartite as indicated hereinabove, the extension that it comprises may be hollow and form a passage for a screw for fixing the piece in the form of a portion of sphere on the plate.
The different positions of the element forming the convex articulation surface with respect to the ground are three in number and are offset, in the sagittal plane, by about 5 mm with respect to one another.
The base is provided with an acromio-coracoid bearing flange which extends substantially in a direction perpendicular to a web of the base in which are provided means for assembly of the element forming the convex articulation surface.
The invention also relates to a complete shoulder prosthesis which comprises a glenoid component as described hereinabove. Such a prosthesis is more comfortable for the patient than the known inverted prostheses.